Research interest in the III-Nitride deep ultraviolet (UV) light emission devices has significantly increased over the past few years. Their potential as a replacement for mercury lamps in several bio-medical, air-water purification, and germicidal systems is one of the key drivers for this research. Due to the transparency requirements, the substrate choices for the sub-300 nm AlGaN deep UV surface emission devices, such as light emitting diodes (LEDs), are limited to either single crystal sapphire or AlN. Currently available bulk AlN substrates typically have a strong absorption band for wavelengths around 280 nm which limits their use for ultraviolet subtype C [UVC] LEDs. Sapphire, due to its high UV transparency, is an excellent substrate choice at the deep UV wavelengths. However, its thermal conductivity is only 0.35 Wcm−1C−1 as compared to AlN substrates which have a thermal conductivity of 2.85 Wcm−1C−1. The lower thermal conductivity results in a high thermal impedance and hence substantially lower DC saturated currents. Simulations show that the thermal impedance can be significantly reduced by incorporating AlN buffer layers with thicknesses in excess of 10 μm over the sapphire substrates. However, when such thick buffer layers are deposited over sapphire using the conventional metalorganic chemical vapor deposition (MOCVD), they crack due to the stress which arises from the thermal expansion coefficient and the lattice mismatch. Moreover, the typical growth rates for AlN MOCVD over sapphire at growth temperatures around 1200° C. are only 0.3-0.5 μm/h. This leads to excessively long times for growing the thick AlN buffers which are required for the low-thermal impedance.
Currently several research groups are actively developing low-defect density AlN substrates to improve the power-lifetime performance of the deep UV LEDs. In the one of the prior art, a new air bridge assisted high-temperature (1500° C.) lateral epitaxy approach to deposit 12 μm thick high-quality AlN layers over SiC substrates as templates for the DUV LEDs. More recently, it has also reported the growth of low dislocation thick AlN layers over grooved SiC substrates for deep UV device epitaxy. Instead of pulsing the metalorganics, a very high growth temperature (1500° C.) with air-bridge assisted CVD growth was used. The 1500° C. growth temperature enabled them to achieve a lateral grow.
Significant progress has also been made in the development of III-Nitride deep ultraviolet (UV) light emitting diodes (LEDs) grown on sapphire substrates using AlGaN multiple quantum well (MQW) active regions. Milliwatt power DUV LEDs for the UVA, UVB and the UVC regions on sapphire substrates with AlGaN multiple quantum well (MQW) active regions have been reported. This progress was largely based on the advancements integrated in the first reported deep UV LEDs demonstrating sub-milliwatt output power. The key to the demonstration of these devices was based on three technical advancements. First, was the use of pulsed atomic layer epitaxy (PALE) to improve the quality of the buffer AlN layer. PALE deposited AlxGa1-xN/AlyGa1-yN short-period superlattices were also inserted between the buffer AlN and the n-contact AlGaN layer to control the thin-film stress, thereby mitigating epilayer cracking. Finally, a p-GaN/p-AlGaN heterojunction contact layer was used to improve hole injection.
In these first generation UVLEDs, under a cw-pump current of 20 mA, the average output powers for state-of-the-art 300 μm2 UVC LEDs are about 1 mW. Due to the poor thermal conductivity of the sapphire substrates, these powers quickly saturate at pump currents around 40-50 mA.
Although the first generation deep UV light emitting devices represent a potential solid state replacement source for more traditional mercury based lamps, these devices suffer from premature performance degradation. Under cw-bias conditions, at 20 mA pump current, the on-wafer device lifetimes (50% power reduction) are only about 50-100 h. 100 h. Their output powers exhibited a fast reduction (˜10% reduction in output power after several hours) followed by a slower decrease (˜50% reduction in output power after 10-100 hours) during on wafer testing. Flip-chip packaging of these devices with heat sinks increased the lifetime to approximately 1000 hours for a pump current density of 200 A/cm2. These studies have shown the initial fast decrease to be both current and temperature dependent and this decrease is related to a device burn-in that creates small, localized alternative current paths, reducing the diode efficiency. At this time it is unclear whether this phenomenon is related to surface states on the mesa sidewalls, or localized regions within the diode active area. It has also been demonstrated that the slow degradation is strongly dependent on the junction temperature which increases with applied bias (joule heating) resulting in the increase and activation of the non-radiative recombination centers. This degradation is a very strong function of the cw-pump current density
The key reasons for this power/lifetime limitation are the dislocations in the active region and the excessive heating due to the poor thermal conductivity of sapphire. Many commercial applications, however, require the output powers and lifetimes to be significantly better than the best values reported to date. Therefore, the lifetime of deep UV LEDs increases significantly if the number of defects in the active area is reduced, and the thermal conductivity of the substrate material is increased to reduce the overall thermal impedance. An increase in device lifetime by approximately 10× was previously observed for 365 nm UV LEDs associated with reducing the defect density from 108 cm−2 to 107 cm−2 and lifetime was also shown to be inversely proportional to junction temperature.
As such, a need exists for high power, stable and highly efficient ultraviolet (UV) light emission devices and method for fabricating the same.